Hydraulic systems have enjoyed wide-spread use in a variety of fields, such as industry, construction, and the like, for such things as raising heavy loads, supporting loads, and raising and supporting work platforms. Hydraulic systems are used widely on such pieces of equipment as front-end loaders, fork trucks, snorkel trucks, tractors, and the like. To a large extent, the systems used are single action hydraulic systems. Single action hydraulic systems are those systems in which hydraulic fluid is introduced into a hydraulic cylinder on one side only of the piston member of that hydraulic cylinder. Introduction of the hydraulic fluid forces the piston in a first given direction and the weight of the load being raised by the piston causes the piston to move in the opposite direction when the fluid that has been introduced to the cylinders is allowed to escape. Although other more elaborate hydraulic systems are utilized in other areas and for other purposes, this basic single action hydraulic system is primarily the type used for lifting and supporting weights as described above.
In this type of a system, particularly when the load being raised is a heavy load or when the work platform being raised and held in its raised position is one on which workmen will remain, a high degree of danger exists as a result of any possible failure in the hydraulic system. In the single action type hydraulic system, should the hydraulic fluid be allowed to escape, the load or platform would drop instantly with potentially very serious results. In the ordinary single action hydraulic system, any break in the hydraulic fluid feed lines, which of necessity are flexible and of a non-rigid, relatively vulnerable material, would cause the load or platform to drop instantly.